This invention relates to an improved catalyst useful for the catalytic isomerization of xylenes and conversion of ethylbenzene. More specifically, it relates to a catalyst composition comprising a crystalline aluminosilicate zeolite, a bismuth component, a Group VIII metal component, and an inorganic oxide matrix.
Isomerization of xylenes is industrially performed by the steps, in suitable combinations, of isomerizing an aromatic hydrocarbon feedstock containing mainly xylene isomers, separating a specified xylene isomer, normally para-xylene, from the resulting isomerization reaction mixture, and recycling the mixture left after separation. It is industrially significant in this case, for an increased efficiency of the isomerization reaction and a reduced cost of production, to adjust the composition of the xylene isomers in the isomerization reaction product as closely as possible to the thermodynamic equilibrium composition, and to inhibit side-reactions such as the decomposition of xylenes (particularly, by hydrogenation of the benzene ring, dealkylation of a methyl group, and transalkylation).
Many methods for isomerizing xylenes have been suggested in the past and many of them involve the use of a crystalline aluminosilicate zeolite-containing catalyst. Crystalline aluminosilicates generally referred to as zeolites, may be represented by the empirical formula: EQU M.sub.2/n O.multidot.Al.sub.2 O.sub.3 .multidot.xSiO.sub.2 .multidot.yH.sub.2 O
in which n is the valence of M which is generally an element of Group I or II, in particular, sodium, potassium, magnesium, calcium, strontium, or barium, and x is generally equal to or greater than 2. Zeolites have skeletal structures which are made up of three-dimensional networks of SiO.sub.4 and AlO.sub.4 tetrahedra, corner linked to each other by shared oxygen atoms. The greater the proportion of the SiO.sub.4 species to the AlO.sub.4 species, the better suited the zeolite is for use as a component in isomerization catalysts. Representative zeolites having such high proportion of SiO.sub.4 include mordenite and the ZSM variety. In addition to the zeolite component, certain metal promoters and inorganic oxide matrices have been included in isomerization catalyst formulations. Examples of inorganic oxides include silica, alumina, and mixtures thereof. Metal promoters such as Group VIII or Group III metals of the Periodic Table, have been used to provide a dehydrogenation functionality. The acidic function can be supplied by the inorganic oxide matrix, the zeolite, or both.
A commercially viable isomerization process is one that concurrently meets the following objectives. First, the process must exhibit high xylene isomerization activity and, second, it must produce the desired product without a significant loss of xylenes. This loss is a result of undesired side-reactions, involving hydrogenation of the aromatic ring, hydrogenolysis, demethylation, and particularly disproportionation and transalkylation.
Another factor of importance in a xylene isomerization process is the effect that ethylbenzene has on the entire isomerization and xylene recovery loop. When ethylbenzene, which is normally present in 8 carbon atom aromatic fractions, is present in appreciable quantities in the feed to the isomerization process, it will accumulate in the loop unless it is excluded from the feed or converted by some reaction in the loop to products which are separable from xylenes by means tolerable in the loop. Ethylbenzene can be separated from the xylenes of boiling point near that of ethylbenzene by extremely expensive "superfractionation". A more desirable method of eliminating the ethylbenzene is through a conversion reaction taking place simultaneously with the isomerization reaction of the xylenes. It is preferable that this ethylbenzene conversion reaction be a deethylation reaction producing benzene and ethane rather than a disproportionation reaction to benzene and diethylbenzene. The deethylation reaction preserves more xylenes and produces a high quality benzene-containing reaction product.
It has now been found that, if a catalyst is formulated with the components, and in the manner set forth hereinafter, an improved process for the isomerization of non-equilibrium mixed xylenes containing ethylbenzene is obtained.